Abstract: The system performances of Mosaic mode in SAR are described in the paper. It describes briefly the characteristics of the Mosaic mode and the differences between the Mosaic mode and the scan and the sliding-spotlight mode. Then it analyzes in detail the steering geometry and the azimuth rotation angle. It also deduces the expressions of the Doppler frequencies and the azimuth resolution for the individual target. Besides, it simulates and calculates the Doppler history and the corresponding azimuth resolutions in the different azimuth of the target. The results provide the technical basis for the Mosaic mode system design.